Irish rail authorities have said it could take around three months to repair the damage and get services back to normal, but there has been speculation that it could take as long as 11 months.

Wendy Blundell, from the Institute of Civil Engineers, described the collapse as “serious”.

“Probably three months is optimistic at this stage,” she said. “An inspection will take some time and then the remedial work is to take place. It depends if further damage has happened to the bridge and any of the other spans.

“I would have thought three months is fairly optimistic, and will probably take a bit longer than that.”

The first Monday-morning commuter passengers were faced with delays of up to 30 minutes yesterday as they were bussed between Drogheda and Dublin.

With thousands of Northern Ireland travellers using the service every week, there have been calls for greater clarity over when the service will be available again.

“Passengers who use the Enterprise Service need confirmation on how long it will take for the bridge to be repaired and services to return to normal,” said Aodhan O’Donnell, head of transport at the Consumer Council.

“This is particularly important for those who buy multi-journey tickets in advance.

“The collapse of the railway bridge near Malahide is an extraordinary situation and we are thankful that there were no injuries or deaths as a result.

“However, there is no doubt that it will cause severe disruption for passengers and commuters who rely on the Enterprise service to travel to and from Dublin.”

Meanwhile, there was further travel misery for one trainload of passengers to Dublin yesterday morning after the Enterprise service broke down en route to the Republic.

Around 100 commuters were left temporarily stranded at Lisburn after the 8am service to Dublin broke down.

Translink blamed a “mechanical failure” for the breakdown, which added a delay of around 45 minutes to the journey. The passengers aboard the train were transferred to another train to continue their journey, which departed from Lisburn shortly after 9am.

Passengers on a return service to Belfast also found themselves waiting at Drogheda for around 45 minutes last night following a delay in the bus connection from Dublin.

Translink have said that services to Dublin generally can expect a 20 to 30-minute delay on average in passengers' journey times.

In Dublin last night, Irish Transport Minister Noel Dempsey ordered Iarnrod Eireann to carry out an urgent safety review of all viaducts and bridges which run across open water on the Republic’s rail network.

And the rail company last night revealed that a preliminary investigation had identified erosion of the seabed as a possible reason for the collapse of a 20-metre section of the railway viaduct near Malahide.

Investigators are now focusing on “recent and significant erosion of the sea-bed” as inspections of the viaduct in 2006 and 2007 found it was structurally sound.

Heavy rainfall, coupled with low tides, could have seen water pressure increase on the bridge which could have caused the collapse.

“It is believed that in a relatively short time frame, possibly in recent weeks, that a small breach occurred in a causeway plateau within the seabed. This would have resulted in changes to water flow, with increased water pressure on the area,” the company said.

“Recent low tides, coupled with major rainfall on Wednesday, would have seen the volume and speed of water flowing out of the estuary increasing, causing water pressures to increase, with ultimately the forces of water pressure widening the breach quickly.

“The effect on the causeway plateau and sea bed would ultimately result in the sudden and catastrophic undermining of the pier supports from below water level, resulting in the collapse of the pier on Friday evening.”

Thousands of commuters are facing disruption for at least the next three months, with services to Balbriggan, Skerries, Rush, Drogheda and intercity services to Belfast affected.

Iarnrod Eireann's investigation will also examine tidal issues in the estuary, rainfall and climatic issues and inspection and maintenance procedures.

The company confirmed the viaduct was inspected twice last week, and that it stood over those inspections. A full bridge inspection was also carried out in October 2007, with the next scheduled inspection for October.

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